In the Fischer-Tropsch process, a synthesis gas containing carbon monoxide and hydrogen is reacted in the presence of a catalyst, which is typically a cobalt- and/or iron-containing composition. The process may be effected using one or more fixed catalyst beds or using a moving catalyst, for example a slurry of the catalyst in a hydrocarbon liquid. The product hydrocarbon liquid is separated from the residual gas. The reaction may be carried out in a single pass or part of the residual gas may be combined with fresh synthesis gas and recycled to the Fischer-Tropsch reactor. Any residual gas which is not recycled to the Fischer-Tropsch reactor for further reaction is here termed tail gas. The tail gas contains some light hydrocarbons, e.g. paraffins including methane, ethane, butane, olefins such as propylene, alcohols such as ethanol, and traces of other minor components such as organic acids, in addition to unreacted hydrogen and carbon monoxide. It will generally also contain some carbon dioxide, which may be present in the synthesis gas fed to the Fischer-Tropsch reaction and/or is formed by side reactions. Possibly, as a result of incomplete separation of the liquid hydrocarbon product, the tail gas may also contain a small proportion of higher hydrocarbons, i.e. hydrocarbons containing 5 or more carbon atoms. These components of the tail gas represent a valuable source of fuel.
Steam reforming is widely practised and is used to produce hydrogen streams and synthesis gas for a number of processes such as ammonia, methanol and the Fischer-Tropsch process.
In a steam reforming process, a desulphurised hydrocarbon feedstock, e.g. natural gas or naphtha, is mixed with steam and passed at elevated temperature and pressure over a suitable catalyst, generally a transition metal, especially nickel, on a suitable support, for example alumina, magnesia, zirconia, or a calcium aluminate cement. In the steam reforming process, any hydrocarbons containing two or more carbon atoms that are present are converted to carbon monoxide and hydrogen, and in addition, the reversible methane/steam reforming and shift reactions occur. The extent to which these reversible reactions proceed depends upon the reaction conditions, e.g. temperature and pressure, the feed composition and the activity of the reforming catalyst. The methane/steam reforming reaction is highly endothermic and so the conversion of methane to carbon oxides is favoured by high temperatures. For this reason, steam reforming is usually effected at outlet temperatures above about 600° C., typically in the range 650° C. to 950° C., by passing the feedstock/steam mixture over a primary steam reforming catalyst disposed in externally heated tubes. The composition of the product gas depends on, inter alia, the proportions of the feedstock components, the pressure and temperature. The product normally contains methane, hydrogen, carbon oxides, steam and any gas, such as nitrogen, that is present in the feed and which is inert under the conditions employed. For Fischer-Tropsch synthesis, it is desired that the molar ratio of hydrogen to carbon monoxide is about 2 and the amount of carbon dioxide present is small.
In order to obtain a synthesis gas more suited to Fischer-Tropsch synthesis, the primary reformed gas may be subjected to secondary reforming by partially combusting the primary reformed gas using a suitable oxidant, e.g. air or oxygen. This increases the temperature of the reformed gas which is then passed adiabatically through a bed of a secondary reforming catalyst, again usually nickel on a suitable support, to bring the gas composition towards equilibrium. Secondary reforming serves three purposes: the increased temperature resulting from the partial combustion and subsequent adiabatic reforming results in a greater amount of reforming so that the secondary reformed gas contains a decreased proportion of residual methane. Secondly the increased temperature favours the reverse shift reaction so that the carbon monoxide to carbon dioxide ratio is increased. Thirdly the partial combustion effectively consumes some of the hydrogen present in the reformed gas, thus decreasing the hydrogen to carbon oxides ratio. In combination, these factors render the secondary reformed gas formed from natural gas as a feedstock more suited for use as synthesis gas for applications such as Fischer-Tropsch synthesis than if the secondary reforming step was omitted. Also more high grade heat can be recovered from the secondary reformed gas: in particular, the recovered heat can be used to heat the catalyst-containing tubes of the primary reformer. Thus the primary reforming may be effected in a heat exchange reformer in which the catalyst-containing reformer tubes are heated by the secondary reformed gas. The use of oxygen as an oxidant rather than air gives further benefits because no inert nitrogen is introduced into the synthesis gas. This means that recycle of CO2 which can be easily absorbed from the syngas in the absence of nitrogen or recycle of unreacted FT reaction tail gas are both feasible and increase the feed gas conversion efficiency to FT liquids. Examples of such reformers and processes utilising the same are disclosed in for example U.S. Pat. Nos. 4,690,690 and 4,695,442.
It has been proposed in WO 00/09441 to employ a reforming process wherein the feedstock/steam mixture is subjected to primary reforming over a catalyst disposed in heated tubes in a heat exchange reformer, the resultant primary reformed gas is then subjected to secondary reforming by partially combusting the primary reformed gas with an oxygen-containing gas and bringing the resultant partially combusted gas towards equilibrium over a secondary reforming catalyst, and then the resultant secondary reformed gas is used to heat the tubes of the heat exchange reformer. In the aforesaid WO 00/09441 carbon dioxide was separated from the product, before or after use thereof for the synthesis of carbon containing compounds, and recycled to the reformer feed. In one embodiment described in that reference, the recycled carbon dioxide was part of the tail gas from a Fischer-Tropsch synthesis process, and was added to the natural gas feedstock prior to desulphurisation of the latter.
U.S. Pat. No. 5,733,941 describes a Fischer-Tropsch process wherein the synthesis gas is produced in an autothermal reformer. The Fischer-Tropsch tail gas is combusted and used to drive a power turbine. The heat from the reformer is passed through a plurality of heat exchangers and the recovered heat is used to raise steam and to pre-heat the reformer hydrocarbon feed gas, Fischer-Tropsch synthesis gas feed and the Fischer-Tropsch tail gas. Although the tail gas is used for power generation, there is still the need for high-pressure steam generation from the heat of the reformer.
U.S. Pat. No. 6,172,124 describes a so-called gas-to-liquids process in which the Fischer-Tropsch tail gas is used to fuel a gas turbine which powers the air compressors used in the process. The synthesis gas is also made in an autothermal reformer in which air and steam is reacted with the hydrocarbon feed gas to generate a syngas mixture containing nitrogen, carbon monoxide and hydrogen. The heat generated in the reformer is recovered from the syngas stream and used to generate steam.
When secondary reformed gas is used to heat the tubes of the heat exchange reformer in which the primary reforming reaction takes place, i.e. when a gas-heated reformer (GHR) is used for the production of synthesis gas, the heat from the reforming reaction is recovered efficiently without the need for high-pressure steam generating plant This is in contrast with the operation of a conventional autothermal reformer, as described for example in U.S. Pat. Nos. 5,733,941 and U.S. Pat. No. 6,172,124 where the heat from the synthesis gas product stream must be recovered in a system of heat exchangers and used for steam generation. Whilst the use of a GHR for production of synthesis gas offers the potential for reducing the steam generation plant required, the overall power requirements of the gas-to-liquids process usually require that steam generation plant is provided for generation of power by means of steam turbines etc. Such steam generation plant may be fuelled by the Fischer-Tropsch tail gas, supplemented with another fuel source, e.g. natural gas which could otherwise be used to generate synthesis gas. Clearly the use of supplemental fuel reduces the overall carbon efficiency of the gas-to-liquids process and the necessity to provide steam generation adds to the cost of the plant.
EP-A-1197471 describes a process for the production of synthesis gas, suitable for feeding to Fischer-Tropsch processes, whereby a hydrocarbon feedstock, e.g. natural gas is reacted with steam and/or oxygen and at least part of any steam requirement is provided by heat exchange against exhaust gas from a gas turbine driving an air separation unit supplying at least part of the oxygen requirement in synthesis gas production. Whereas the gas turbine is fed by a combustible fuel gas that may contain a portion of the Fischer-Tropsch process tail gas, the production of synthesis gas does not comprise primary reforming of the hydrocarbon feedstock/steam mixture over a catalyst disposed in heated tubes in a heat exchange reformer, subjecting the resultant primary reformed gas to secondary reforming by partially combusting the primary reformed gas with an oxygen-containing gas and bringing the resultant partially combusted gas towards equilibrium over a secondary reforming catalyst, and then using the resultant secondary reformed gas to heat the tubes of the heat exchange reformer.
We have found that a gas-to-liquids process employing such a synthesis gas production process may be made more efficient when the Fischer-Tropsch tail gas is used to fuel a gas turbine for power generation.